A federal appeals court yesterday struck down the Bush administration's strategy for reducing smog, ruling that the Environmental Protection Agency's rules for forcing state cleanups do not meet requirements of the Clean Air Act.

Judge Judith W. Rogers, writing for the three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit, said, "EPA has failed to heed the restrictions on its discretion set forth in the Act." The court ordered the agency to come up with another enforcement plan.

Smog is produced by nitrogen oxides reacting with other chemicals to create ground-level ozone, particularly in the summer months when the sun is hottest and brightest. Other major sources of the pollution are motor vehicle exhaust, gas vapors and chemical solvents.

In the stratosphere, ozone occurs naturally and is a beneficial shield against harmful ultraviolet rays.

EPA in 2004 issued new smog standards requiring 474 counties, with about 159 million people living in them, to reduce pollution of smog-causing ozone and improve the air enough to meet federal health standards. The agency gave them three to 17 years to do so.

The appellate panel said that was too long. It also said the agency was allowing "backsliding" by states when it should have been directing them to order new emission controls on industrial plants, more public transportation, tougher vehicle-inspection programs or cleaner-burning gasoline. Under the law, states that do not meet the standards risk losing federal highway dollars.

EPA spokeswoman Jennifer Wood said yesterday that the agency will review the panel's decision to determine whether it will ask the full appeals court to hear the case.